Potentiometer



Jarl- 5, 1954 DE WITT T. VAN ALEN ET AL 2,665,355

POTENTIOMETER Filed Feb. 7, 1952 e OR PERCENT ROTAUON P16600 .2 .3 .4 .5 .e .7 .9 I

o .O5 .8 PER CENT .lo -OO ecomfmw ERROR DEGREES 5.4 .2o 7'2 l5 JNVENTORS Y 28 |8 DE wlTT T. VAN ALEN F167 E R T HAROLD G. FooTE Patented Jan. 5, 1954 UNITED STATES PATENT OFFICE POTENTIOMETER D e Wittl T. Van Alex1, Delavan, and Harold G. Foote, Fontana Wis., assignors to The George W. Borg Corporation, Chicago, Ill-;, a corporation of Delaware Application February 7, 1952, Serial No. 270,364 12 Claims'. (Cl. 20L-56) The present invention relates in general. to Fig. 3k is a diagrammatic View showing the remulti-turn potentiometers, or variable resistors, lation between the slider contact and the helix inl which the resistance element has the form of a turns at opposite ends oi the resistance element; compound-helix, andthe object oi the invention is Figs. 4 and 5 shoe,v removable stl-ips which are a 116W and mIJIOVed @Otenlometel Of this 5 mounted on the cylinder such as shown in Fig. l character. in a modiiied form of the invention;

Theinvention maybe considered as an Lnprove- Fig. 6 is a graph showing curves produced by ment on the potentiometer disclosed` in. the U. S. plotting loading error against shaft rotation, the patent to Gibbs et al., No. 2,405,321, grantedJ anucurves also showing the correction in degrees for ary 24, 1950. lo any-per cent of shaft rotation; and

The Gibbs et al. potentiometer comprises a Fig. '7 is a circuit drawing which will be used resistancev element having preferably ten turns in the explanation of the invention. in the major helix, a lead screw concentric there- Except for the improvements introduced by the with and having the same pitch, a nut or support present invention the potentiometer is substanthreaded on the lead screw, said support carryl5 tia-ily the same as the one disclosed in the Gibbs ing` a slider contact engaging the resistance eleet al; patent previously referred to and consement, and means comprising a coaxial shaft and a quently it will not be necessary to go into great slotted cylinder mounted thereon for rotating the detail regarding its construction and process of support, the leadl screw being effective during manufacture. such rotation to advance the support and contact Referring to Figs. 1 and 2, the reference characaxially to enable the contact to follow the turns ter lo indicates a casing of generally cylindrical of the major helix. form which is molded of some material such as According to one -feature of the invention the Bakelite. The casing is closed at one end by the guiding edge of the cylinder slot is given a formaflanged bearing member l 2, which is molded into tion which advances or retards the rotation of the the. casing whenv it is made. The bearing inemsiider contact relative to the rotation of the opber has an externally threaded portion carrying erating shaft according to a predetermined curve. the nut I3 by means of which the potentiometer The non-linear relation between Contact rotation may be mounted on a panel such as U. and shaft rotation is employed to produce a po- The resistance element is indicated at l5 and tentiorneter in which the relation between the with its terminals IS and Il is also `molded into output voltage and shaft rotation is linear althe casing when it is made. The resistance elethough there is a load in the output circuit. It ment has an insulated core of soft copper wire and may also be used to correct manufacturing errors a helical winding of bare resistance wire, Wound and for other purposes, as will be explained. on the core in spaced turns. The core is formed According to another feature, the slider contact into a major helix and is long enough to make isv enlarged in ther direction parallel to the axis somewhat more than ten complete turns. The of the resistance element, being given a semiturns of thehelix and the turns of the resistance cylindrical shape, for example, and the leadscrew wire wound thereon are held in xed position by is made with a pitch which is slightly different the nioldedcasing. from the pitch of the .major helix of the resistance The open end of the casing it is closed by the element. When the contact is rotated along the end cap IB which may also` be molded of maresistance element the small area of the contact terial Such as Bakelite. The end cap carries a whichy is in engagement with the resistance eleterminal I3 fOr the Sldl' COH'CaC, and S. re-

ment progressively changes or shifts along the movably secured inthe casing by means of screws contact surface, thereby greatly increasing the Such as 20,Fig.2.

useful area of the contact and increasing the life The Operating Shaft Of the POteIl'iOmStel iS lthereof, ind1cated atv 2|, and is rotatably supported in the The foregoing other features of the invenbearing member l2. The Cylinder 22 inside the tion will be described more in detail hereinafter, CaSn iS fixed 011 the flanged @110i Of Shaft 2| by reference being made to the accompanying. drawmeans Of adiSC. Of llisulating material which EleC- ing, in whichtrically insulates the cylinder from the shaft.

Fig 1 is a, pal-tial Vertical section h1-@ugh a The reference character. 23 indicates the lead potentiometer embodying the invention; screw. At one end the lead screw has a cylindri- Fig. 2 is a transverse section on they line 2-2, cal portion 24 by means of which it is xed to Fig. 1; 55 theendy cap 1.9. At the other endthe lead screw has a similar cylindrical portion of reduced diameter which has a bearing in the shaft 2I. The cylinder 22 which is fixed to shaft 2l as previously mentioned is supported at the other end by means of a disc of insulating material which has a bearing on the part 2d of the lead screw.

The nut 25 is threaded on the lead screw 23 and includes a slider contact support 26 which projects through a slot in the cylinder 22. The nut and support are made of insulating material. The support 26 may be square in crosssection, except for a rounded portion where it passes through the slot in the cylinder, and is recessed at the end for reception of the spring 21. This spring is secured to the support 26 by a screw and carries the slider contact 28 which it presses against the resistance element I5.

The Contact spring 21 is electrically connected to the cylinder 22 by means of the conductor 30 which has one end soldered to the contact spring and the other end soldered to the cylinder, the conductor being loosely Wrapped around the cylinder to permit axial movement of the nut 25 and contact spring 21. The cylinder has an annular contact at the left hand end which is engaged by the bifurcated contact spring 3i, whereby the cylinder is electrically connected to the terminal I8.

The heretofore mentioned slot in cylinder 22 has a guiding edge 32 against which the rounded portion of the support 26 is pressed by the spring 34. This spring is fixed to the nut 25 as shown in Fig. 2 and rides along the other edge 33 of the slot. The guiding edge 32 of the slot has an accurately predetermined curved contour as will be subsequently explained. The edge 33 could in most cases be straight but in order to allow a minimum tension in spring 34 it is preferably given a contour which conforms generally to the contour of edge 32.

The end cap I9, lead screw 23, nut 25, cylinder 22, and shaft 2l constitute an assembly which is removable from the casing as a unit.

The potentiometer may be operated by means of a suitable dial mounted on the shaft 2I. Ro-

' tation of the shaft rotates the cylinder 22, which rotates the nut 25 and moves the slider contact 28 along the major helix turns of the resistance element, the stationary lead screw causing the necessary axial movement of the nut to enable the contact 28 to follow the turns. Rotation of the shaft 2l in a, counterclockwise direction as seen from the right in Fig. 1 moves the nut 25 to the left, or toward the terminal I6, until the support 26 engages the stop 36, Fig. 2, which arrests the movement. 'Ihe stop is part of the end cap I9 which is so oriented in the casing that when the shaft and dial have been rotated as far as they will go in this direction, the support 26 having engaged the stop 36, the slider contact 28 will be in engagement with the minor helix coil of the resistance element which is connected to the terminal I6. In this position of the slider contact the resistance of the connection between terminals I8 and I6 is zero, or substantially zero.

If a.calibrated dial is used for rotating the shaft 2l the dial should read zero when the slider vcontact is at the terminal I6. A turn counting dial which is suitable for the purpose is disclosed in the application of Van Alen et al., Serial No. 152,596, filed March 29, 1950. This dial counts turns from 1 to 10 and also shows decimal fractions of a turn.

Rotation of the shaft 2I in a clockwise direction moves the nut 25 and slider contact 28 to the right or toward the terminal I1. Upon rotating slightly more than 3600 degrees in this direction the slider contact 28 passes the minor helix coil which is connected to terminal I1 and after a few more degrees of rotation the support 26 engages a stop (not shown) which is part of the casing I0 and similar to the stop 36 in the end cap, although facing in the opposite direction. It will be noted from the foregoing that the terminals I6 and I1 are slightly more than 3600 degrees apart. This is to insure that the resistance element will have a minimum of ten effective turns, or an angular length of 3600 degrees as measured by the dial.

A potentiometer such as described herein may be used to derive an output potential which is a desired fraction of a given potential which may be referred to as the input potential. Fig. '1 shows a simple circuit arrangement for this purpose. From some source not shown, a voltage E is impressed across the conductors 40 and 4I, which are connected to terminals I6 and I1 of the potentiometer. The resistance R is the total resistance of resistance element I5. The output circuit is connected to terminals I6 and I8 of the potentiometer and is represented as including a load resistance Rr.. The quantity e represents the voltage across RL, or the Voltage between terminals I6 and I8.

It will be seen that voltage e depends on the position of the slider contact 28 on the resistance element I5, or on the angular distance the slider contact is displaced in a clockwise direction from its zero position at terminal I6. The quantity 0 may be used to represent the percentage of full rotation expressed as a decimal fraction. Therefore, in the zero position of the dial the quantity 0 is equal to zero, at 50% of full rotation it is equal to .50, and at full rotation (3600 degrees) it is equal to 1.00.

In a potentiometer having a linear output the voltage e corresponding to any position of the slider contact is expressed by the equation -has some finite resistance the Voltage e is not a linear function of 0, but is ascertained by the equation Examination of Equation 2 shows that when 0 is equal to 1.00, e is equal to 0E and to E, and that when 0 is equal to zero, e is equal to zero. These facts are also evident from inspection of the circuit diagram, Fig. '7.

For other values of 0 the value of e depends on the ratio R/Rr. and is always less than 6E. The difference between e and 0E decreases as the load resistance RL is increased and for very high values of load resistance the output voltage e does not depart very much from linearity.

The relation between e and 0E as 0 is increased from zero to 1.00 is shown by means of two curves in Fig. 6, each curve corresponding to a v3500 degrees.

each value of 6, and the curvel d is constructed byV plotting the respective voltages thus found against the corresponding values of Itv will be seen that with the assumed ratio of -R/RL the departure `ci voltage e from linearity is not great at any point. The error is greatest When 0 is approximately 70% and at .that point is equal to about .15 volts when the input voltage E is G volts. The error expressed as a percentage of the input voltage E is .l5 per cent and is independent of the absolute value of E.

The curve i6 corresponding to a value of .001 for the ratio R/RL is constructed in the same tray as described. RL being larger in proportion to P., than in the case of curve e5, the error is smaller and the curve is flattened out considerably, as is to be expected.

It will be appreciated now that since the error is negative, e being smaller than at all points on the curve except when 9 equals zero or 1.00, it can be corrected by introducing supplenientary clockwise rotation of the slider contact 28 which is cumulative with or additional to the clockwise rotation produced by shaft 2l and measured by the dial. The number ci degrees of :A

supplementary rotation which is required to cor recty any error in volts can be readily calculated yand the results of such calculations are shown at the right of the graph, Fig. 6.

Explaining this further, it will be recalled that the angular length. of the resistance element is When the error is .05 per cent therefore the number of degrees correction required is .05 per cent of 3500 degrees, or 1.8 degrees. An error of .10v per cent requires a correction of .10A per cent or 3600 degrees, or 3.6 degrees. The other Values shown are found in the same way.

Referring now to Fig. l, it will be seen that the guidingv edge 32 of the slot in cylinder 22 has been given a curved contour corresponding to the curve e5 inFig. 6 in order to compensate for the error in the value of the output voltage that would otherwise be present, thereby producing a potentiometer having an output voltage c which isa linear function of the per cent rotation 0,

notwithstanding that the output circuit is not open buthas a finite resistance and draws current. 'When the sha-ft 2l is rotated seven full turns, for example, as indicated by the dial, the slider contactv 28 is rotated seven full turns, and also, due to thecurvature of the guiding edge 32, is rotated approximately 5.4 degrees in. addition,

thereby correcting for the loading error and mak- -ing the output voltagec equal to-HE.

The guiding edge 32 as shown in Fig. 1 has a contour which corrects the potentiometer when `the ratio` of R/RL isl equal to .61.

The same cylinder can be used for different loadings of the outputcircuit, however, sincev` the potentiometers are manufactured withresistance elements which vary in. resistance over a wide range, and it will usually be possible in` any given case to select a potentiometerhaving a resistance R of such value that the ratio R/RL is equal to .01'. It will be 6 understood of course that several standard cylinders may be manufactured each having a guiding edge 32 conformi-ng vto a different value of the ratio-R/Rn.

According to a modiiicaticn of the invention, the slot in the cylinder 22l is enlarged as indicated by the dotted lines in- Fig. 1 two strips 56 and 5l suchl as shownin- Figs. 4 and 5 are attached tothe cylinder by means of screws. The strips have-holes such as 54 and 55 at the ends for the screws andthe cylinder has corresponding tapped holes such as 52 and 53. When the strips are in position on the cylinder they cooperate to form a slot having the guiding edge 32 andan edge 33 which is engaged by the spring 34.

This modica-tion is more' ilegible than the first described modification. It can be used inall situations where standard cylinders 22, Fig. 1, can be used, and is also adapted to talee care of cases where the potentiometer loading' isoutside the range of such standard cylinders. That is, inl cases where the guiding edge of .the cylinder must have a special contour. An advantage oi the removable strip slot construction is that it is possible to change the contour of the guiding edge to adapt the potentiometer to a different loading. This is accomplished by taking out the end. cap, cylinder and shaft uni-t and replacing the strips 5i) and 5|V with a new set having the desired slot contour.

The modilied form of thev invention may also be4 used to correct. departures from linearity due to manufacturing errors and makes it possible to produce extremely accurate potentiometer-s.

As. applied to an ordinary potentiometer not corrected. for loading error, a strip such as 5G is made with ev straight guiding edge 32". rThe strip 5i if provided should also have a straight edge. The potentiometer is assembled complete, mounton a test panel and equipped with a dial. The resistance between terminals i8 and I is then measured with the dial in a plurality of spaced positions positions, for example) by means of a bridge circuit in order to determine the actual departure from linearity such positions. The errors thus found are plotted against percentage .of rotation and a curve is constructed which shows the manufacturing error at any percentage of rotation. This curve may be substantially a straight line, as whenno detectable error is present, but will usually be more or less irregular. If a cyclical error associated with the major helix is present 'the curve may bear some resemblance to a sine curve, crossing the zero line or Y axis once in each 360 degrees of dial rotation.

In actual practice a testing machine is employed which rotates the potentiometer shaft automtically in steps of 36 degrees each, compares the potentiometer resistance with a standard resistance in each position of the shaft, and records the result oi each comparison on a tape in rms of distance above or below a median or zero line. By means of a machine such as described inoividal error curves for pctentiometers can be made very quickly, although the machine is not absolutely essential'.

When the error curve of a potentiometer shows that it needs correction, tl e end cap I9 is rebringing with it the lead screw, cylinder and shaft unit. The strip such as 5o is then removed and replaced by a similar strip the guiding edge 52 oi which has been ground or otherwise modified to conform to the error curve. lThe po tentiometer is then reassembled and the new strip .50 will modify the slider contact rotation relative to rotation of the dial and corerct the manufacturing error that would otherwise be present.

It will be understood that since correction for loading error and manufacturing error are both accomplished by modifying the relation between slider contact rotation and dial rotation it is possible to make a strip such as D having a guiding edge 32 of the proper contour to make both corrections. The two errors should be measured simultaneously, as by testing the potentiometer for manufacturing error with the output circuit of the potentiometer loaded, thereby obtaining the data for constructing a composite curve which is the resultant of manufacturing error and loading error.

The strips 50 and 5| as shown in the drawing are designed to accomplish another object of the invention. As mentioned hereinbefore the terminals I6 and l1 are slightly more than 3600 degrees apart as measured by the dial and as a result the slider contact 28 has not yet reached the terminal Il when the dial indicates ten full turns. This is generally of no consequence, and is ignored in the manufacture of the cylinder 22 shown in Fig. 1. For some purposes, however, it is desirable to have the slider contact at terminal l1 when the dial indicates 3600 degrees, which requires an additional rotation of the slider contact amounting to perhaps two or three degrees.

Inspection of Fig. 4 shows how the additional rotation above referred to its introduced. The guiding edge 32' has been given a curved contour to correct for loading error, but the base of the curve, indicated by the dotted line, is not parallel to the axis of the cylinder when the strip 5u is assembled thereto. On the contrary, the right hand end of guiding edge 32 is advanced several degrees beyond the left hand end, which gives the slider Contact the additional rotation required. The angular length of the line 56 is a measure of the additional rotation thus provided for.

It will be understood that the amount of additional rotation required should be ascertained before the guiding edge 32' is ground to the desired load correcting contour. This is done by setting the dial at 3600 degrees, testing the resistance between terminals l8 and ll, advancing the dial until the measured resistance is equal to Zero, and then reading on the dial the number of degrees it has been advanced beyond 3600 degrees. strip 50 to be used is then cut out along the lines 56.-51, after which the guiding edge 32' may be ground to conform to a curve which has its base at the dotted line. In the case of a potentiometer which is not to be corrected for loading, the guiding edge will be made to coincide with the dotted line.

Referring now to Fig. 3, it has been mentioned that the lead screw 23 has a different pitch than the turns of the major helix in order to shift the Working contact area along the slider contact `28 as the slider contact is rotated along the turns of the resistance element. The lead screw may have more or less turns to the inch than the major helix but as the parts are shown in Fig. 3 the lead screw is assumed to have the finer pitch of the two. The slider contact 2S is shown in full lines on the first turn of the resistance element l5, where it is located when the dial reads zero; and is shown in dotted lines in the position on the last turn of the resistance element which it reaches when the dial reads 3600 degrees. It will be seen that as the dial is rotated from The zero position the snoer contact will move to the right to follow the turns of the resistance element but at a slower rate than the advance of the major helix, due to the finer pitch of the lead screw, with the result that the area of the slider contact which engages the resistance element shifts along the slider contact from left to right. Thus in zero position of the dial the working area of the slider contact is somewhat to the left of the center of the contact, whereas in the farthest advanced position of the dial, when the slider contact is on the tenth turn of the resistance element, the working area is somewhat to the right of the center of the contact. The Wear on the contact is thereby distributed over a substantial part of the contact, and the tendency to wear a groove in the contact surface is eliminated.

The invention having been described, that which is believed to be new and for which the protection of Letters Patent is desired will be pointed out in the appended claims.

We claim:

l. A potentiometer comprising a helical resistance element, a slider contact adapted to travel helically along the turns of said element, means including an operating shaft for driving said contact, and means effective during travel of said contact to change its rate of movement relative to the rate of the operating shaft.

2. A potentiometer comprising a resistance element in the form of a multi-turn helix, a slider contact engaging said element, means including an operating shaft for rotating said contact on the axis of said helix, means responsive to such rotation to move said contact axially to enable it to follow the turns of said helix, and means responsive t0 said axial movement to modify the relation between the number of degrees rotated by said contact and the number of degrees rotated by said shaft.

3. In a potentiometer, a casing, a resistance element having the form of a multi-turn helix supported inside said casing, a slider contact engaging said element a support for said contact rotatable on the axis of said helix, an operating shaft, a device rotated by said shaft and having a guiding edge engaging said support to control rotation of the support and contact, and means responsive to such rotation to move said support axially in sliding engagement with said guiding edge, thereby causing said slider contact to follow the turns of said helix, the said guiding edge having a contour which at different points in the travel of said slider contact causes the number of degrees rotated by the Contact to differ from the number of degrees rotated by said shaft.

4. In a potentiometer, a casing, a resistance element having the form of a multi-turn helix supported inside said casing, a slider contact engaging said element, a support for said contact rotatable on the axis of said helix, an operating shaft, a device rotated by said shaft and having a guiding edge engaging said support to control rotation of the support and contact, and means responsive to such rotation to move said support axially in sliding engagement with said guiding edge, thereby causing said slider contact to follow the turns of said helix, the said guiding edge having a contour which during the travel of said slider' contact causes the number of degrees rotated by the contact to exceed the number of degrees rotated by said shaft by an amount which is sufficient to compensate for the departure from linearity produced by a predetermined load on the output circuit.

5. In a potentiometer comprising a resistance element in the form of a multi-turn helix, a. slide-i.l contact engaging said element, a support for said contact, means including an operating shaft supported on the axis of said helix for driving sairl support and Contact along the turns of said helix, said means also including a driving element connected to said shaft, said driving element having a guiding edge in continuous engagement with said support and having a contour which uniformly accelerates the rotation of the and Contact relative to rotation of said shaft.

6. In a potentiometer comprising a resistance element in the form of a multi-turn helix, a slider contact engaging said element, a support for said contact, means including an operating shaft supported on the axis of said helix for driving said support and contact along the turns of said helix, said means also including a driving element ccnnected to said shaft, said driving element having a guiding edge in continuous engagement with said support, the portion of said guiding edge which engages said support when said Contact is on the last turn of said helix being angularly advanced relative to the portion of said guiding edge which engages said support when said contact is on the rst turn of said helix.

7. In a potentiometer as claimed in claim 5, a terminal connected to the resistance element at a point on the last turn thereof which is beyond the point to which the contact would be advanced by a number of rotations of said shaft equal to the number of turns in said helix, the number of degrees by which said contact is advanced in its travel from the ilrst to the last turn of said helix being sufficient to bring said contact to the point on the last turn at which said terminal is connected.

8. In the manufacture of multi-turn potentiometers in which the slider contact is mounted for simultaneous rotary and axial movement and is rotated by means of a shaft and a guide fixed thereto along which the contact support slides during rotation, the method of correcting a potentiometer for loading error which consists in ascertaining the loading error at different points in the rotation of said shaft, and making said guide with a curved Working surface which engages said support and advances the support and contact a suicient amount at said points relau tive to rotation of the shaft to correct said error.

9. In the manufacture of multi-turn potentiometers in which the slider contact is mounted for simultaneous rotary and axial movement and is rotated by means of a shaft and a guide xed thereto along which the contact support slides during rotation, the method of correcting a potentiometer for manufacturing error, which for rotating said nut on said consists in ascertaining the error at different points in the rotation of said shaft, and grinding or otherwise shaping the working surface of said guide to advance or retard the rotation of said support and Contact at said points the proper amount relative to rotation of said shaft to correct said error.

l0. In a potentiometer, a casing, a resistance element having the form of a multi-turn helix supported inside said casing, a slider contact having a portion of its contact surface engaging said element, a lead screw disposed on the axis of said helix, a support for said contact including a nut threaded on said lead screw, and means lead screw, the pitch of said lead screw being near enough to the pitch of said helix to cause said contact to follow the helix for a plurality of turns but differing therefrom sufficiently to cause successive turns of said element to be engaged by different portions of said contact surface.

11. In a potentiometer, a casing, a resistance element having the form of a multi-turn helix supported inside said casing, a slider contact having a contact surface, extending parallel to the axis of said helix, means supporting said contact with a working area of its contact surface in engagement with said element, means for rotating said supporting means and contact about the axis of said helix, and means responsive to such rotation to simultaneously impart axial movement to said contact to cause the contact to follow the helix turns, said last means comprising a lead screw having a pitch which diiers sufficiently from the helix pitch to cause a substantial movement of said working area along said contact surface as the slider contact moves around the turns of said helix.

l2. A potentiometer as claimed in claim 1l, wherein the lead screw has more threads per inch than the helix, causing the Working area to shift along the contact surface in the saine direction as the axial movement of the contact as a whole.

DE WITT 'I'. VAN ALEN. HAROLD G. FOOTE.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,495,321 Gibbs et al Jan. 24, 195o 2,554,811 Bromberg et al. May 29, 1951 FOREIGN PATENTS Number Country Date 254,382 Switzerland Apr. 30, 1948 

